In current lighting applications, energy efficiency is a more and more important subject.
One possible way to reduce the energy consumption of a lighting system is to switch off or dim the lights when no persons are present, and conversely to switch on the lights when someone is present. In order to do this, the presence of any people in the relevant space has to be detected (e.g. to detect whether there is anyone present in a certain room or area of a room). The presence of a person in a space may be described as occupancy. Different types of occupancy sensors or presence sensors are currently in use. Most of these sensors are motion sensors and use a passive infrared (PIR) sensor to detect motion.
Another way of detecting the presence of a person is to use an active sensing technique whereby one or more waves are emitted into the space in question and presence is detected based on echoes received back. One technology suitable for use in active sensing is ultrasound (US). An active ultrasound presence detector sends out a signal in the form of a series of bursts of acoustic waves (or a continuous wave) at an ultrasonic frequency, e.g. 40 kHz. The sensor then uses the echoes it receives back from the environment to determine whether there is presence in that environment, e.g. in a room. Different methods can be used for this, for example Doppler shift measurements, time-of-flight measurements, and/or moving target indicator (MTI) processing.
When performing active measurements in an environment, the measurement may be compromised by another signal source in the same space that emits a signal with similar characteristics as the presence detector. For example it has been shown that when a keychain or set of keys is rattled, it emits sound including a component in the ultrasound spectrum and this can lead to a false positive when performing a measurement at the same time. As another example, the sensor in question may experience interference from another nearby ultrasound sensor transmitting at the same or even a different moment in time, depending on such factors as output sound pressure level, time between bursts from the two sensors, and distances between the two sensors.
Such false positives could lead to lights being turned on when no occupant is present. Alternatively if the sensitivity of the sensor is decreased to try to avoid this, then small but genuine motions of a person may be missed. Interference or other disturbance may also be an issue in other sensing applications.